The present invention relates to a method for picture compression, as can be used for example in video or TV coders or decoders with motion-compensated prediction, and also to a corresponding device for picture compression.
Future digital TV decoders shall be able to receive not only the CCIR or PAL picture format but also high resolution signals, so-called HDTV signals (High Density Television), and to reproduce them in a reduced size on conventional screens. One problem in the decoding of high resolution video signals is the enormous memory requirement for the large-format internal picture memory.
A crucial part of contemporary video coding standards, such as, for example, the standards belonging to the H.26 or MPEG family, is so-called motion-compensated prediction. In this connection, the predecessor picture is in each case stored as a reference picture by the coder and the decoder, and only the differences with respect to the successor picture are transmitted, in order to reduce the coding extent. In addition, in accordance with the picture motion, blocks each comprising 16×16 pixels, for example, are displaced from the predecessor picture in order to ensure the best possible prediction of the successor picture.
In the decoding of HDTV signals which are to be reproduced on conventional screens, it is endeavoured, for cost reasons, not to use a reference picture memory for storing the individual pixels in the HDTV format, but rather merely a reference picture memory having a reduced storage capacity. In particular, the intention to use a reference picture memory which is only suitable for storing the picture data in the conventional SDTV format and, accordingly, has only a quarter of the storage capacity required for storing HDTV picture data. Different variants for compressing the reference pictures to be stored in the reference picture memory are known for this purpose and will be explained in more detail below with reference to FIG. 4.
FIG. 4 illustrates the basic construction of a TV or video decoder with motion-compensated prediction, as is used for example in digital TV sets. As has already been mentioned above, when using motion-compensated prediction, only the differences between two successive pictures are transmitted. In the case of the arrangement shown in FIG. 4, the difference values received in coded form are firstly fed to a variable length decoder 8, in order to convert the difference values into code words having uniform bit length. These code words are subsequently fed to a block 9 for carrying out an inverse quantization and a block 10 for carrying out an inverse discrete cosine transform (IDCT). In order to obtain the actual picture, the difference values are added to the values of the individual pixels of a reference picture which is stored in a reference picture memory 5 and correspond in particular to the preceding picture. Conversely, the pixel values of the instantaneous picture that are thus obtained, as shown in FIG. 4, are stored again in the reference picture memory 5, in order to store the instantaneous picture as a new reference picture.
Since, for cost reasons, the reference picture memory 5 is only configured for storing picture data in the conventional SDTV format, the picture data present downstream of the adder shown in FIG. 4 or the corresponding HDTV picture, must be compressed. In order to achieve this, the respective picture can be subsampled by a unit 1 for example with the factor ¼, with the result that the memory outlay for storing the HDTV picture compressed in this way is reduced. The picture data read from the reference picture memory 5 then have to be correspondingly decompressed by a unit 6 in order to obtain the original HDTV format again.
Generally, each picture is processed in blocks, and, by way of example, each block may comprise 16×16 pixels. In order to ensure the best possible prediction of the successor picture, blocks are displaced relative to the predecessor picture in accordance with the picture motion. For this reason, the picture data of the individual blocks that are read from the reference picture memory 5 are fed to a unit 7 for carrying out the motion compensation and for carrying out a corresponding interpolation. The principle of motion-compensated prediction is generally known in principle, and so it need not be discussed in more detail at this point.
Situated at the output of the video decoder shown in FIG. 4 is a switch 11, which can be used to effect a changeover between HDTV and SDTV picture reproduction. As has already been explained above, the picture data are present at the output of the adder shown in FIG. 4 (after the motion-compensated prediction explained above has been carried out) in the HDTV format. However, the desired SDTV format can be obtained from this with the aid of a subsampler 12, which subsamples the picture data or the corresponding pixels in accordance with the factor ¼.
With the aid of the subsampling of the HDTV picture carried out by the unit 1, as is described for example in H. Sun, “Hierarchical Decoder for MPEG Compressed Video Data”, IEEE Trans. Consumer Electronics, Vol. 39, No. 3, 1993, pages 559-564, although the memory outlay required for storing the reference picture in the reference picture memory 5 can be reduced, the picture quality is nonetheless impaired on account of the reduced resolution. Instead of this subsampling in the space domain, subsampling in the frequency domain can also be carried out by the unit 1, as is described for example in A. W. Jonson, T. Sikora, T. K. Tan and K. N. Ngan, “Filters for Drift Reduction in Frequency Scalable Coding Schemes”, ELECTRONIC LETTERS, 17 Mar. 1994, Vol. 30, No. 6, pages 471-472. However, a subsampling method of this type is not very suitable for detailed picture structures.